The present invention generally relates to electrophotography and more particularly, to an electrophotographic apparatus which may be applied, for example, to a copying machine, reader-printer or the like.
In FIG. 1, there is schematically shown a side sectional view of a conventional electrophotographic apparatus as applied to an electrophotographic copying machine, which generally includes a photoreceptor drum 1 having a photosensitive or photoreceptor layer 1a for example, of a selenium photosensitive material applied onto the outer peripheral surface of the drum 1 and rotatably provided generally at a central portion for rotation in a direction indicated by the arrow, while a corona charger 4 for uniformly charging the photoreceptor layer 1a, an optical system having a light source S for illuminating an original document O placed on a transparent platform P and a lens L for forming an electrostatic latent image of the original document O on the photoreceptor surface 1a, a developing device 2 for developing the latent image into a visible toner image, a transfer charger T, and a cleaner blade B, etc. are sequentially disposed around the photoreceptor drum 1 in a known manner.
The developing device 2 further includes a housing 2a, a magnet roll 3 having a developing sleeve 6 adapted to rotate in the direction of the arrow and impressed with DC developing bias voltage V in the range of +150 V to +400 V for eliminating fogging in a resultant image and a magnetic roller M fixedly provided within said developing sleeve 6.
Hereinbelow, operation of the conventional electrophotographic copying machine in FIG. 1 will be described.
The photoreceptor layer 1a of the photoreceptor drum 1 is uniformly charged up to voltages in the range of +700 to +800 V by the corona charger 4 so as to subsequently have the electrostatic latent image of the original document O formed thereon by the exposure through the optical system, and said latent image is then developed into the visible toner image by the developing device 2 in which a dual or two-component developing material composed of electrically conductive iron particle carrier and toner is contained, with the distance between the surface of the developing sleeve 6 and the photoreceptor layer 1a of the drum 1, i.e., the so-called developing gap being set to be in the range of 1 to 5 mm.
Meanwhile, as another prior art arrangement of this kind, there have conventionally been proposed, for example, in U.S. Pat. Nos. 3,890,929, 3,866,574 and 3,893,418, electrophotographic apparatuses utilizing a flying developing method by a mono-component developing material composed only of non-contacting toner.
In the prior art references as referred to above, for effectively charging toner by the corona charger, the thickness of the toner layer is set to less than 0.1 mm so as to be smaller than the distance between the surface of the developing sleeve and the photoreceptor surface of the drum, and the toner layer is kept out of contact with the photoreceptor surface, so that the toner adheres onto said photoreceptor surface by flying.
In the conventional arrangement as described so far, due to the fact that the photoreceptor layers of selenium, organic photo semiconductor, etc. have inferior resistance to rubbing, the time period from maintenance to maintenance for the electrophotographic apparatus is undesirably shortened, and even if the conventional photoreceptor is replaced by a photoreceptor mainly composed of amorphous silicone, there is still a problem that, since the amorphous silicone photoreceptor layer has a small in thickness, with low surface potentials in the range of 200 to 400 V, it is difficult to obtain high image density, contrast and resolution.
Moreover, in the electrophotographic apparatus utilizing the flying developing method of the non-contacting mono-component developing material, it is necessary to keep the toner layer uniformly thin at a thickness less than 0.1 mm, and if the toner layer is apt to contact the photoreceptor surface, the distance between the surface of the developing sleeve and the photoreceptor surface must be set smaller than the thickness of the toner layer, thus requiring a high accuracy difficult to be achieved. Furthermore, since the developing method utilizes the flying of toner, there is a tendency for scattering of toner, with a consequently insufficient resolution of the resultant images.